Process for the production of a finely divided,non-gelatinous silicic acid especially suitable as a filler for natural and synthetic rubber and other elastomers



May 20, 1969 H. J. MAAT ET AL PROCESS FOR THE PRODUCTION OF A FINELY DIVIDED, NON-GELATINOUS SILICIC ACID ESPECIALLY SUITABLE AS A FILLER FOR NATURAL AND SYNTHETIC RUBBER AND OTHER ELASTOMERS FIG. I.

FOR: T 80 C.

Sheet of4 2 A D B c Hmin.) I

PH 5 8 7 50 I00 I50 2 00 250 500 p E H F G H INVENTORS HENDRIK JAN MAAT JOHANNES EBREGT AT TORNE Y May 20, 1969 H. J. MAAT ET AL 3,445,189

PROCESS FOR THE PRODUCTION OF A FINELY DIVIDED, NON-GELATINOUS SILICIC ACID ESPECIALLY SUITABLE AS A FILLER FOR NATURAL AND SYNTHETIC RUBBER AND OTHER ELASTOMERS Filed March 5, 1966 Sheet & 0154 ..8 (Q N .9 m A .E' E d 3-; 2 0 H Q h O m cl 1 o o s u m D n q 8 l n m m'\ 2 N I N LL Q 3 2 2 F? a. II N ON 5; i Q Q 8" g E m '2" 9 8 2 s 2 2 .9 a 2 s 2 2 or- N N 4 0 2 28 b. 3?. 0.0 S 2 l 2 =3 3 a 8 2 .5 Tn u INVENTORS HENDRIK JAN MAAT JOHANNES EBREGT A TTORNE Y May 20, 1969 H. J. MAAT ETAL 3,445,189

PROCESS FOR THE PRODUCTION OF A FINELY DIVIDED, NON-GELATINOUS SILICIC ACID ESPECIALLY SUITABLE AS A FILLER FOR NATURAL AND SYNTHETIC RUBBER AND OTHER ELASTOMERS Filed March 5, 1966 Sheet 3 of 4 FIG. 4.

OR: pH 7.4 and T: 80C.

I 1' (min) I so I00 150 2'00 2'50 (93 SiOz) FOR= pH=9.0 and T=BOC.

' Hmin.) 50 I00 150 200 250 300 6 6 INVENTORS HENDRIK JAN MAAT BY JOHANNES EBREGT ATTORNEY May 20, 1969 PROCESS FOR THE PRODUCTION OF A FINELY DIVIDED, NON-GELATINOUS SILICIC ACID ESPECIALLY SUITABLE AS A FILLER FOR NATURAL AND SYNTHETIC RUBBER AND OTHER ELASTOMERS Filed March 5, 1966 H. J. MAAT ET AL 3,445,189

Sheet of 4 FIG. 5.! o I 2 c.

IO l4 I8 22 20 FOR= pH: 8.2 and T= 80C.

I Hmin.) I

FOR pH 82 and T: C.

- Hmin.) 50 I60 I50 260 550 INVENTORS FIG. 52

HENDRIK JAN MAAT JOHANNES EBREGT ATTORNEY United States Patent 3,445,189 PROCESS FOR THE PRODUCTION OF A FINELY DIVIDED, NON-GELATINOUS SILICIC ACID ES- PECIALLY SUITABLE AS A FILLER FOR NATU- RAL AND SYNTHETIC RUBBER AND OTHER ELASTOMERS Hendrik J. Maat, Diemen, and Johannes Ebregt, Beemster,

Netherlands, assignors to Koninklijke Zwavelzuurfabrieken v/ h Ketjen N.V., a limited-liability company of the Netherlands Filed Mar. 3, 1966, Scr. No. 531,414 Claims priority, application Netherlands, Mar. 5, 1965, 6502791 Int. Cl. C01b 33/00 US. Cl. 23-182 Claims ABSTRACT OF THE DISCLOSURE In the process for producing finely divided silicic acid by simultaneously adding solutions of an alkali silicate and a strong mineral acid to water at a temperature between 70 C. and 90 C., then acidifying the reaction mixture and then separating, washing and drying the silicic acid formed: a pH of 7 to 9 is maintained during the additions of the alkali silicate and strong mineral acid solutions to the Water, and'the addition of such solutions to the water is continued until the concentration of Si0 in the reaction mixture has attained a value in the range between 2.5% and 5.0%, by weight, and, as a function of the pH and temperature of the reaction mixture, corresponds to a point within FIG. 7.

This invention relates to a process for the production of non-gelatinous silicic acid in finely divided form, and particularly to an improved process of the type in which alkali silicate solutions and strong mineral acids are introduced simultaneously into an aqueous liquid to form a reaction mixture, the reaction mixture is acidified after completion of the addition, and the silicic acid formed is separated, washed and dried.

Such methods of production are known, for example, in Belgian Patent No. 629,129, in which the solutions of alkali silicate and acid are fed simultaneously into an alkaline aqueous solution, the viscosity of the aqueous medium is maintained low during at least 30% of the total precipitation period, and the feeding of the reac tion components is interrupted only when the viscosity, after passing through a maximum, has dropped to a Value which is less than double the initial viscosity. This is achieved also by working at a temperature between 80 C. and 90 C. and a pH between 10 and 12, preferably at about 10. The addition of both solutions is continued until the content of SiO is between 50 and 150 g./l.

It has been found that when the SiO is to be used for various purposes, for example, as a filler for natural and synthetic rubber and other elastomers, the surface area of the silicic acid and also the oil absorption are of vital importance. The surface area of the silicic acid is determined by means of the nitrogen absorption method of Brunauer, Emmett and Teller (J.A.C.S. 60, 309, 1938).

3,445,189 Patented May 20, 1969 ice The oil absorption, which is a measure of the porosity, is the quantity in cc. of linseed oil per g. of silicic acid just sufficient to be able to form a small ball of the silicic acid (modified Gardner-Coleman oil absorption test). It has been found that in the production of silicic acid the surface area and oil absorption do not vary in parallel, but that a kind of satisfactory comprise must be attained between surface area and oil absorption.

It has been found that such a satisfactory compromise can be realized, and this is the essence of the present invention, if water is taken as the aqueous liquid, during the simultaneous introduction of the alkali silicate and acid solutions into the water at a temperature in the range of C. to C. the pH is maintained in the range of 7-9, and the introduction of the alkali silicate and acid solutions is continued until a SiO concentration of 2.5-5% in the reaction mixture has been reached.

Extensive investigations have further indicated that, if a finely divided silicic acid is to have good reinforcement properties for rubber, it must have a surface of to 250 mF/g. and an oil absorption of more than 2 cc./ g.

During simultaneous proportioning or introduction of the alkali silicate and acid solutions, the oil absorption for selected values of pH, temperature, and concentration depends on the proportioning time, and is found to initially increase with time, pass through a maximum, and then decrease as the proportioning time is extended. To reach the desired compromise between surface area and oil absorption, the proportioning should be ended before the maximum oil absorption has been reached.

Although it has been proposed in German Patent No. 946,433 to slowly react solutions of alkali silicate and acid with intensive agitation particularly at temperatures between 80 C. and 100 C. while the pH is maintained at a value of 6-8 during the entire precipitation period, nothing whatever is said about the duration of the proportioning or the final concentration to be achieved. In this process, there is no acidification at the end of the reaction, so that the sodium among other things, cannot be washed out completely from the end product. Although considerable amounts of sodium may be tolerated in silicic acid for pharmaceutical purposes, such material is not suitable as a filler for natural and synthetic rubber and other elastomers.

In German Patent No. 966,985, Example II is a process in which alkali silicate and acid solutions are simultaneously proportioned or introduced, at a temperature of 70 C. and a pH of 5, into a 7.5-percentual soduim chloride solution. Here apparently there is acidification to a low pH after completion of the reaction. Examples I and II operate at room temperature, while, in Example IlI, no temperature at all is given and evidently was not considered to be important. Further, in Examples I and IV a pH of 8.5 is adopted, while in Example II no pH range is given and it is only mentioned that ammonia is liberated, from which it may be concluded that the reaction takes place in the alkaline range. In all examples it is emphasized that an indifferent electrolyte must always be present in a concentration of at least 1% at the beginning of the precipitation. he proportion,-

3 ing rate, and hence also the proportioning time, has, according to German Patent No. 966,985, no appreciable effect on the quality of the product obtained, which is quite contrary to what is the case in the process according to the present invention.

In the process according to the present invention, any soluble silicate, for example, sodium or potassium silicate, may be used. However, for practical reasons, dilute waterglass with a SiO /Na O ratio of 3.3/1 is preferred. Likewise any strong mineral acid may be used, but sulfuric acid is preferred.

It is clear that in the course of the process the silicic acid concentration of the mixture gradually increases. After a certain time, jellying occurs, and thereafter a finely divided precipitate forms. The SiO concentration (C in percent, by weight, in the reaction mixture at time t (in minutes) is expressed by the formula:

where: P is the weight increase in grams of the reaction mixture per minute over 100 times the number of grams of SiO,, proportioned per minute; and Q is the quantity (V) of water in grams over 100 times the number of grams of SiO proportioned per minute.

Q is thus a measure of the rate of the SiO concentration build-up. Small values of Q correspond to a high initial rate of SiO concentration build-up. P is a measure of the dilution of acid and waterglass in the reaction mix; ture.

Further, at constant pH the following formula applies:

m Constant where C =the percentage by weight of (for example) H 80 in the acid,

F =the added quantity of acid in grams per minute (feed rate),

C =the percentage by weight of Si in the alkali silicate solution,

F =the added quantity of alkali silicate solution in grams per minute (feed rate).

P and Q can also be expressed as follows:

With the aid of the five quantities, pH, T, P, Q and t, the process can be described exactly.

There will now be given a simpler formulation, which is more directly connected with the range in which one works.

It has been found that, in the pH range 7-9 at a certain temperature, there belongs to each pH a first approximation of an optimum range for the final Si0 concentration (C at which the desired compromise for the resulting finely divided silicic acid is attained. For 80 C. these values are:

at pH 7: C =2.53.5% at pH 8: C =3.25-4.25% at pH 9: C =4.0-5.0%

This implies that for selected values of P and Q at a cer- 4 tain temperature the proportioning time must lie between the times which, according to the formula 13 05- coincide with extreme values for C at a certain pH.

This is true only within certain limits of the values for P and Q. From the definitions of P and Q it follows that these quantities depend on V, C F and P In practice, one usually works with waterglass solutions having a C of 15-27% and with moderately dilute sulfuric acid solutions having a C of 525% When using too dilute a waterglass solution, the desired final SiO concentration cannot be attained. Starting with waterglass and acid solutions of a certain concentration, one can then adjust to the desired values for P and Q by means of the feed rate P and F The process according to the present invention is preferably carried out at values of P between 0.05 and 0.25 and values of Q between 2 and 20. For each temperature there can be defined a three-dimensional area in the orthotomic coordinate system X =time t, Y=C Z=pH, and within which the end point of the proportioning must fall in order to obtain finely divided silicic acids with good reinforcement properties for rubber. Such three-dimensional area for a temperature of C. is shown on FIG. 1 and the projections of the area on the three orthogonal planes are shown on FIG. 2. The optimum area herein is expressed by the polyhedron defined by the points A B C D E F G H having the following coordinates:

The area is limited by two planes of constant pH, to wit, plane A B C D at pH=7 and plane E F G H at pH=9, respectively; by two planes A B F E and D C G H, in which, at a pH from 7 to 9, C varies from 2.5 to 4% and from 3.5 to 5%, respectively, and by two planes A D H E and B C G F, the latter of which indicates a limitation for the Si0 concentration build-up rate.

As has been mentioned, the above described area is valid only at a constant temperature of 80 C. but it is also possible to prepare good, finely divided silicic acids at other temperatures between 70 C. and C. This is illustrated by FIGS. 3a and 3b.

In FIG. 3a there is indicated the area I J K L within which, at pH=8.2 and at temperatures T between 70 C. and 90 C., the final SiO concentration C (in percent) in the reaction mixture must lie in the process according to the invention.

In FIG. 3b there is indicated the area I J" K" L within which, at the same pH as in FIG. 3a and at temperatures T between 70 C. and 90 C. the proportioning time t in minutes must lie in the process according to the invention.

It is seen from FIGS. 3a and 3b that, at higher temperatures, good silicic acids are obtained at a lower final SiO concentration and at shorter proportioning times. At lower temperatures, of course, the reverse applies.

From FIGS. 1 through 3b it is seen that the optimum area A B C D E F G H indicated in FIG. 1 for 80 C., shifts at higher temperatures toward lower final SiO concentration and shorter proportioning times, while the reverse is true at lower temperatures. In practice, however, it is favorable to maintain a temperature of 80 C.

For further illustration there is shown on FIG. 7 the C range .according to the process embodying the invention as a function of pH and temperature. The coordinates of the points defining the octahedron shown on FIG. 7

are:

totem moo. mu enaenmsowentnea The octahedron is limited 'by the plane Bb Cc Dd, wherein C :5% the plane Tt Uu Vv, where in C =2.5 the plane Aa Gg Uu Vv Kk, wherein the pH=7, the plane Cc Dd Ee Rr Nn Mm, wherein the pH=9, the plane Aa Bb Cc Mm Ll Kk, wherein the temperature is 70 C., the plane Gg Ff Ee Rr 5.? Tt Uu, wherein the temperature is 90 C., and two more planes which are fully determined by the joints defining the six previously identified planes.

FIG. 4 is a cross-section of the polyhedron of FIG. 1 at pH=7.4, and the area T U V W on FIG. 4 is the area of good silicic acids. FIGS. 5a and 5b are both cross sections of FIG. 1 at the pH=8.2. The area -P Q R S on FIGS. 5a and 5b is the area of good silicic acids. One works preferably in this area.

FIG. 6 is a cross section of FIG. 1 at the pH=9.0. The area E H G H is the area of good silicic acids.

What this amounts to is that, according to the process of the present invention, at a certain pH in the range from 7 to 9, at a certain temperature in the range from 70 to 90 C., a P in the range from 0.05 to 0.25, and a Q in the range of 2 to 20, the proportioning time falls within a certain range which, at 80 C., is between 6 and 270 minutes. At this time, more than 95% of the silicic acid is precipitated. Subsequently the pH of the reaction mixture is reduced, for example, with sulfuric acid, to a value lower than 4. This treatment is necessary to fix the desired final surface of the silicic acid. It is essential that this acidification proceed rather quickly. This rapid acidification can be effected, for example, by adding acid, after termination of the simultaneous proportioning, at a much greater speed than during the simultaneous proportioning, and "by intensive agitation. Another method of effecting rapid acidification consists in letting the reaction mixture as a whole run out of the reacton vessel into a boiler containing the quantity of acid required for the acidification and having means for effecting intensive agitation. Lastly, the silicic acid is filtered off and washed with sufficient water, for example until the Na O content is less than 1%.

The washed filter cake is dried on plates in a drying cabinet at 140 C. for 10 to 20 hours, or in an atomization dryer. In this way finely divided silicic acid having a surface area of 100250 m. g. and an oil absorption of over 2.0 cc./g. is obtained.

This silicic acid can be used excellently as a reinforcing filter for synthetic and natural rubbers, and is particularly suitable as a filler for so-called transparent rubbers.

For good rubber properties the following characteristics are adhered to:

Mooney plasticity (non-vulcan- (ASTM-D 1646-62)- ized mixture) 100 to 140. Scorch time, minutes (non-vul- (ASTM-D 1646-62) canized mixture) 410.2.

Hardness, Shore '(ASTMD 676-59T) Modulus 300, kg./cm. (ASTM-D 412-62T) Drawing strength, kg./om. (ASTM-D 412-62T) Elongation at rupture, percent (ASTM-D 412-621) Tensile strength, l '.g./cm. (NEN5603) 45.

Wear, mm? (DIN53516) 180.

Transparence Visual good.

The silicic acid according to the invention can further be used advantageously as a vehicle for insecticides and aromatics, as an anticaking agent, as additive to asphalt and other bituminous materials for the improvement of their mechanical properties, as a thickening agent and as a thixotropic agent for many liquids, as an antiskid agent for paper or plastic, and as an antiblocking agent, for example, for polyethylene, whereby sheets of this plastic can no longer stick together. Another interesting application of the produced silicic acid is as an agent for the conversion of liquids into powders. Silicic acid can absorb, for example, up to 400% of its own weight of concentrated sulfuric acid without losing its powder form, while the characteristic sulfuric acid action is retained.

A number of illustrative examples of the process according to the invention are given below:

Example 1 In a rustfree tank of a capacity of 20 l. and provided with an agitator, 6000 g. of water are introduced and heated as quickly as possible to a temperature of C.

While stirring intensively, there are proportioned simultaneously dilute waterglass solution (concentration: 10.1% SiO and 3.06% Na O) at a rate of 58 g./minute and dilute sulfuric acid (concentration: 19% H 50 at a rate of 14 g./minute. P and Q are thus 0.12 and 10.24, respectively. During this addition the temperature is maintained at 80 C. and the pH is constant at 7.4. The addition is terminated after a total of 55 minutes. In the finely divided suspension thus obtained the SiO; concentratino is 3.3%.

Then the reaction mixture is poured into a dilute sulfuric acid solution so that the pH is rapidly l wered to 2.5. The finely divided silicic acid is filtered oif and washed with water. Following this, the filter cake is dried on plates at a temperature of 140 C. for 15 hours. A finely divided silicic acid having a surface of 230 m. g. and an oil absorption of 2.3 cc./ g. is obtained.

Examples 2 to 45 In the following Table I -a number of variants of Example 1 are given. The examples are presented in groups carried out according to the same concentration build-up, but for various proportioning times. For greater clarity, Example 1 is also included. The examples are indicated also on FIGS. 3-6 and identified by the corresponding numbers of the examples as listed in Table I.

The silicic acids which have good reinforcement properties for rubber (surface -250 m. /g., and oil absorption greater than 2.0 cc./ g.) are marked with a plus sign in the last column of Table I, and it will be seen that these examples are all included within the ranges according to the invention.

Q, t,min. 05, SA, A, percent mfi/g. mlJg.

TABLE I Cw, z percent percent T., 0. pH V, gram Fw, Fa, gjmin. gJmin.

Ex. No.

5551555500500 05 200050000850600 0 5 50009008 56034.87944fiocp woudzmnvd 025136%050359%5m5m04682681 1 1. 1 1111.211 1 11 12 11 1 1 000555333999000555533332223555333335555555555 111 .111 .111 .1111 .111 0088 9&22 8888 222888844 8888888888 Parts (by Wei ht) silicic acid and C. The properined in this manner are ixture) 192 ine ine

Mooney plasticity (non-vulcanized m Scorch time (minutes) (non-vulcanized mix- 40 Triethylam Mercaptobenzimidazol Diphenyl guan Di-2-benzothiazy1 disulfide 0.8 Ethylene glycol 3.0 Sulfur Silicic aci h 24 hours after the admixture of the agltatlin t ere rubber, the mixture is vulcanized at 150 tely 6 Same ties of the rubber platelets obta pomt, dilute waterglass (concentration 9% S10 and as follows 2.72% Na O) at a rate of 15 kg./minute and dilute sulring inten approxima In the above Table I, the values given under the column headed SA are of surface area measured by the method of Brunauer, Emmett and Teller; and the values given und by the Modified Gardner-Coleman Oil Absorption Test.

Example 46 In a tank of stainless steel provided with an agitator ing a capacity of 1 in. there are introduced 135 are simultaneously introduced ic acid (concentration 15% H 80 at a rate of 4.125 kg./minute.

mine

der the column headed 0A are of oil absorption deterand hav kg. of water of 80 C. Du

fur

I I n u em a e fwe HMDET mmd a Am ii mmm n a m b W mmuw.

s Mm a. mm w. m m o m m nmHew q ummm3 m a. H

t tt n zrn hm mm re ghooncm u e s D ut e er VImm Negative the last column of with a plus sign 1n nc acid. The finely divided slliclc aci is filtered Wear mm 3 and washed with water. The resulting filter .cake is well g ':III::IIIIIIII: Very good Total impression and an exlt tempera- 65 In the following Table II are given the pertinent values for a number of variants of Example 46, and for EX- Slhclc. acld havmg a S.urface. of 293 ample 46 itself which is there identified as No. 47. Those m. /g. and an 011 absorption of 3.5 cc./ g. is obtalned.

examples WhlCh are earned out according to the same con- 1c1c am is mlxed lnto a so-called transparent centration build-up are arranged in groups on Table II 1c1c acid-rubber in com ositiolr and all of Examples 47 to 58 are plotted as pomts on FIG. g p 5b which are identified by corresponding numerals. The

Parts (by weight) acids which have good reinforcement properties for rub- Styrene-butadiene rubber No. 1502 100 ber are marked ive ZlIlC oxide 2 Table II, and are shown on FIG. 5b to be within the d 2 ranges according to the invention.

A finely divided The sil' rubber mixture on a rubber roller. The sil ture has the follow ic aci dilute sulfu homogenized in a grinding device and then dried in an atomization dryer at a spray pressure of atm., an entrance temperature of 500 C ture of C.

mix

Act

Stear TABLE II F., kg./ F., kg./ 0., 0., 0., s A Ex. No '1, 0 pH V, kg min. min Percent Percent P Q, t, min. Percent mfllg. mlJg Rubber Properties I II III IV V VI VII VIII IX X 192 4. 5 76 65 250 620 46 145 S.g. 101 3. 8 67 63 130 535 39 215 s. 129 4. 2 72 39 200 615 45 165 g. 127 4. 1 75 46 220 605 48 155 S.g. 84 3. 9 67 58 160 520 33 190 s. 110 3. 9 74 32 210 655 50 175 g. 109 a s 73 35 210 640 47 170 g. 79 3. 6 69 43 190 595 39 185 S. 88 3. 7 70 28 190 640 43 185 m. 77 3. 8 67 26 175 645 37 195 m. 75 3. 7 66 30 180 630 195 s. 79 3. 8 67 37 185 605 38 185 S.

In the portion of the above Table II relating to Rubin which her Properties, the column headings have the following meanings:

I-Mooney plasticity II-Scorch time (minutes) IIIHardness, Shore IVModulus 300 kgJcm.

V--Drawing strength kg./cm.

V1Elongation at rupture, percent VII-Tensile strength kg./cm. VHF-Wear mm. IXTransparency (visual) XTotal impression Although specific embodiments of the invention have been described in detail herein, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. The process for the production of iinely divided silicic acid 'by simultaneously adding solutions of an alkali silicate and a strong mineral acid to a quantity of an aqueous liquid at a temperature between 70 C. and 90 C., acidifying the reaction mixture after completion of the addition of said solutions, and then successively separating, washing and drying the silicic acid formed; wherein said aqueous liquid is water, during the addition of said solutions of alkali silicate and strong mineral acid to the water a pH in the range of 7 and 9 is maintained, and the addition of said solutions to the water is continued until a time when the concentration of S102 in the reaction mixture has attained a value which is in the range between 2.5% and 5.0%, by weight, and which, is as a function of the pH and temperature of the reaction mixture, corresponds to a point within threedimensional figure number 7 in which X, Y and Z are orthogonal coordinates along which there are respectively plotted values of pH of the reaction mixture, the concentration of SiO; in the reaction mixture, in percent, by weight, and the temperature of the reaction mixture in degrees centigrade.

2. The process according to claim 1; wherein said process is conducted in accordance with the formula:

C is the SiO concentration in percent, by weight, in

the reaction mixture,

2. is the time, in minutes, during which said solutions of alkali silicate and strong mineral acid have been simultaneously added to the water,

P is the rate of weight increase of the reaction mixture, in grams per minute, over times the rate at which SiO is proportioned, in grams per minute, and

Q is the quantity of water, in grams, over 100 times said rate at which Si0 is proportioned, in grams per minute, and in which P and Q respectively have values in the ranges between 0.05 and .25 and between 2 and 20.

3. The process according to claim 1; wherein the temperature of said reaction mixture is maintained at approximately 80 C., and the addition of said solutions of alkali silicate and strong mineral acid to said water is continued for a time which, in connection with the pH and temperature of the reaction mixture correspond to a point within three-dimensional figure number 1.

4. The process according to claim 1; wherein the temperature of the reaction .mixture is maintained at about 80 C., and the addition of said solutions of alkali silicate and strong mineral acid to the water is continued for a time in the range between 6 and 270 minutes.

5. The process according to claim 4; wherein the pH of the reaction mixture is maintained at 8.2 and the addition of said solutions of alkali silicate and strong mineral acid to said water is continued for a time within the range between about 15 and 230 minutes so as to attain a final concentration of Si0 in the reaction mixture of from about 3.4% to 4.4%, by weight.

:6. The process according to claim 1; wherein said acidifying of the reaction mixture reduces the pH of the latter to below 4.0.

7. The process according to claim 6; wherein said acidifying of the reaction mixture is eifected by pouring the latter into a quantity of acid with intensive agitation.

8. The process according to claim 1; wherein said washing of the silicic acid formed is continued until the Na O content thereof is less than 1%, by weight.

9. The process according to claim '1; wherein said drying of the silicic acid formed is effected on plates at a temperature of C. to C. for a period of 10 to 20 hours.

1 1 12 10. The process according to claim 1; wherein said FOREIGN PATENTS drying of the silicic acid formed is effected in an atomi- 1,168,874 4/ 1964 Gummy zation dryer.

R f e Cit d OSCAR R. VERTIZ, Primary Examiner.

UNITED STATES PATENTS 5 ARTHUR GREIF, Assistant Examiner.

2,940,830 6/1960 Thornhill 23182 US, Cl, X,R

3,235,331 2/1966 Nauroth et al 23-482 106309 

